Luminescence of Agrellite Specimen from the Kipawa River Locality

Abstract

Using steady-state luminescence measurements, the luminescence spectra of Ce³⁺, Pr³⁺, Nd³⁺, Sm³⁺, Eu³⁺, Dy³⁺, Er³⁺ and Yb³⁺ for the agrellite sample from the Kipawa River region have been measured. The emission spectra of Eu³⁺ and Dy³⁺ next to those of Sm³⁺ and Pr³⁺ have been measured for characteristic excitation conditions. The most efficient luminescence activator in the studied sample was Ce³⁺. This ion was also a sensitizer of Pr³⁺, Sm³⁺, Eu³⁺, and Dy³⁺ luminescence.

1. Introduction

Agrellite is a rather rare mineral and is usually found in pegmatite lenses and pods or in mafic gneisses in a regionally metamorphosed agpatic alkali rock complex. It occurs with eudialyte, britholite, aegirine, as well as miserite, vlasovite, calcite, fluorite, clinohumite, gittinsite, norbergite, zircon, biotite, phlogopite, galena, and quartz. The most known localities of it are the Sheffield Lake complex, Kipawa River, Villedieu Township, Québec (Canada), Dara-i-Pioz massif, Alai Range, Tien Shan Mountains (China), (Tajikistan), the Murun massif, southwest of Olekminsk, Yakutia, (Russia) and Wausau complex, and finally, Marathon Co. in Wisconsin (USA). Among the minerals from the Kipawa River complex, only britholite, fluorite, calcite, miserite, and zircon are predisposed to exhibit fluorescence from lanthanide ions. However, no such reports are known so far. The knowledge of agrellite luminescence comes from websites only [1,2,3], and from the Gorobets and Rogojine review book [4]. According to these sources, agrellite has a dull pink color or a bright pink color under shortwave ultraviolet (SW UV) or longwave ultraviolet (LW UV, respectively. The following emission centers have been observed [4]: Ce³⁺, Dy³⁺, and Sm³⁺ for samples from the Kipawa River; Fe³⁺, Mn²⁺, Eu²⁺, and presumed Nd³⁺ for samples from the Yakutia; and of Ce³⁺ and Mn²⁺ for specimens from the Dara-i-Pioz. The latest results of spectroscopic investigation of agrellite samples from Dara-i-Pioz (Tajikistan) and Murun massif (Russia) shows luminescence from Ce³⁺ and EPR spectra of Mn²⁺ [5].

An attempt was also made to synthesize sodium calcium silicate with an initial composition such as agrellite [6]. These materials were doped with Mn²⁺ (1%, 2%, and 3%), Ce³⁺ (0.5%), and Tb³⁺ (4%), but as a result of synthesis, multiphase nanoparticles composed of wollastonite 2M CaSiO₃, devitrite Na₂Ca₃Si₆O₁₆, and cristobalite SiO₂ was formed.

The current paper presents the preliminary results of a study of the luminescence properties of agrellite NaCa₂Si₄O₁₀F specimens from the Kipawa Alkaline Complex, Québec, Canada.

Agrellite crystallized in triclinic, space group P(−1), and unit cell parameters are: a = 7.759 (2) Å, b = 18.946 (3) Å, c = 6.986 (1) Å, α= 89.88°, β= 116.65°, γ= 94.34°, Z = 4 [7]. All atoms in the agrellite lattice are present in general positions and the Wyckoff site of all atoms is denoted as 2i. The crystal structure of agrellite consists of two different NaO₈ distorted cubes polyhedra, two CaO₅F octahedra (hereinafter referred to as Ca1B and Ca2A), and two CaO₆F₂ polyhedra (named as Ca1A and Ca2B) coupled with two different Si₈O₂₀ chains [7]. The mean of Ca–O and Ca–F distances are smaller for CaO₅F octahedra and equal for Ca1B and Ca2A, respectively 2.381 Å, 2.192 Å, 2.367 Å, and 2.201 Å than CaO₆F₂ and equal for Ca1A and Ca2B, respectively 2.574, 2.402, 2.615 Å and 2.454 Å (Figure 1a,b).

2. Materials and Methods

The agrellite sample studied here takes the form of fine lath-shaped aggregates measuring from a few to over 100 mm in length. It has a greenish-white color (Figure 2a) which changes to lavender-pink under mid UV (Figure 2b).

Rare earth elements (REE) concentrations in the studied agrellite crystal were measured by the ISP-MS method at ACMA Lab (Canada), and the results are listed in

Table 1. Concentration of some transition elements in the studied agrellite sample.

Contents (ppm)
Y	La + Lu	Ce	Pr	Nd	Sm	Eu	Gd
>2000	>2000	>2000	1174	>2000	1034	136	1265
Contents (ppm)
Tb	Dy	Ho	Er	Tm	Yb	Mn	Fe
221	1419	282	772	93	458	1454	<2000

. Steady-state fluorescence measurements for ions shown in the chemical analysis were performed using a Jobin-Yvon (SPEX) spectrofluorimeter FLUOROLOG 3-12 (Jobin-Yvon, Grenoble, France SPEX at room temperature using a 450 W xenon lamp, a double-grating monochromator, and a Hamamatsu 928 photomultiplier (Hamamatsu Photonics, Shizuoka, Japan). The wavelength range for emission and excitation spectra was from 250 to 900 nm, and the resolution no lower than 1 nm. For such conditions, the emission and excitation spectra of Ce³⁺, Pr³⁺, Sm³⁺, Eu³⁺, Dy³⁺, and Er³⁺ in the UV–Vis range were recorded. The emission spectra of Nd³⁺, Yb³⁺, and Er³⁺ also in the NIR range were measured utilizing a Dongwoo Optron DM711 monochromator (MaeSan-Ri, Opo-Eup, GwangJu-Si, Gyeonggi-Do, Korea) coupled with a Hamamatsu 928 photomultiplier (Hamamatsu Photonics, Shizuoka, Japan), and an InGaAs detector (Hamamatsu Photonics, Shizuoka, Japan) or PbS detector (Hamamatsu Photonics, Shizuoka, Japan) depending on the spectral region. An InGaAs diode laser (Appolo Instruments, Irvine, USA) emitting infrared radiation at 975 nm and an AlGaAs diode laser (CNI, Changchun, China) emitting at 808 nm were used as continuous wave excitation sources.

3. Results

In the studied agrellite specimen, the minimum content of lanthanide ions as a potential luminescence activator can be estimated as 1.0% (Table 1). If the distribution of these ions is uniform, then these ions are present in every 2.5 unit cell. The maximum distance among 4f ions is not less than 17.5 Å and not more than 47.25 Å.

Using steady-state fluorescence measurements, the emission of the following lanthanide ions were measured: Ce³⁺, Pr³⁺, Nd³⁺, Sm³⁺, Eu³⁺, Er³⁺, and Yb³⁺. Despite the significant Mn content, the emission spectra of Mn²⁺ were not measured. Unlike the [4] data, no emission from Fe³⁺ was obtained.

3.1. Nd³⁺, Yb³⁺, and Er³⁺ Luminescence

The emission of Nd³⁺ was measured as a group of three multiples (Figure 3). The most intense was measured at 1046, 1060, 1064, 1078, and 1092 nm and corresponds to the ⁴F_3/2 → ⁴I_11/2 transition. Another group of lines at 879, 885, 893, 905, and 917 nm is associated with the ⁴F_3/2 → ⁴I_9/2, transitions and the last with lines at 1313, 1335, and 1340 nm-with the ⁴F_3/2 → ⁴I_13/2 transitions. The emission spectrum of Yb³⁺ was not very intense (inset of Figure 3) and band 979 nm corresponds to the transition ²F_5/2 → ²F_7/2.

It was expected that the measurement of distinct emission lines of Er³⁺ would be possible due to the significant content of this ion and its high luminescence efficiency. However, the emission bands of Er³⁺ corresponding to the ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions and usually measured at a 500–600 nm range were not clearly visible for steady-state luminescence measurements using a xenon lamp excitation, contrary to time-resolved measurements [8]. It has previously been verified [9] that for steady-time measurements using a xenon lamp, the most convenient excitation for Er³⁺ is λ = 377 nm. However, for the studied agrellite specimen, only a weak emission band at 543 nm was measured (Figure 4). In the NIR range, the distinct emission band at 1533 nm (1554 nm, 1512 nm, and 1488 nm) was measured as a result of the ⁴I_13/2 → ⁴I_15/2 transition (Figure 5). Using laser excitation, emission bands of ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2, and ⁴F_9/2 → ⁴I_15/2 transitions were also obtained (inset in Figure 5). However, the intensities of these bands at 520 and 550 nm were very weak, not only in comparison with the Er³⁺ emission in the NIR range but also with the Sm³⁺ and Pr³⁺ emission lines, although the samarium or praseodymium concentration was not much higher than the Er content (Figure 6).

3.2. Ce³⁺ Fluorescence

The cerium content in the studied agrellite sample was high. Electric-dipole 4f–5d transitions are even and spin allowed with the oscillator strength at half-width and short luminescence decay time. Cerium Ce³⁺ is often added to synthetic crystals because of its well-known properties as an efficient sensitizer of luminescence. Luminescent materials doped with Ce³⁺ can efficiently absorb excitation energy. A very intensive emission band at 388 nm has been measured (Figure 7). The zero phonon line (ZPL) position was designated in the point of spectral overlap of the excitation and emission curves, i.e., at λ = 337 nm (29,673 cm⁻¹). The Stokes shift ∆S, as the energy difference between absorption/excitation and emission maxima of transition between the lowest 5d and the 4f ground states, is equal to 1567 cm⁻¹. This value is not very high, probably owing to the high coordination number around Ce³⁺ and the long Ca–O distance [10,11]. For the lowest Raman frequency of agrellite 331 cm⁻¹, the Huang-Rhys parameter S is equal to S = 2.8, so electron-phonon coupling may be recognized as intermediate.

On the excitation spectra, two bands at 284 nm and 321 nm were recorded, so the crystal-field splitting 10 Dq of 5d level was estimated as 4059 cm⁻¹. The upper excited level of Ce³⁺ is T_2g and the lower is E_g. The 10 Dq parameter for Ce³⁺ usually has a value in the 5000–10,000 cm⁻¹ range. For the studied crystal it was slightly smaller because the Ca–O length in agrellite is larger than in other Ca–minerals, for example fluor-apatite. The emission band of Ce³⁺ is nearly symmetric, although it was fitted to two Gaussian components with maxima at 28,023 cm⁻¹ and 25,819 cm⁻¹ (R² = 0.998) that can be discerned, (Figure 8), which correspond to the transition terminated on the ²F_5/2 and ²F_7/2 levels.

The intensity of the Ce³⁺ emission band at 388 nm measured for λ_exc = 284 nm was almost 1.5 times higher than those observed for λ_exc = 321 nm, despite the power of the xenon lamp is lower at λ = 284 nm than at 321 nm. This may indicate that a significant part of the excitation energy at λ_exc = 321 nm in the studied mineral transferred to other luminescence centers.

In [5] for the agrellite sample from Dara-i-Pioz, the emission and excitation spectra of Ce³⁺ have been presented. The Ce-content in this sample was 1160–1263 ppm, distinctly less than for the Kipawa River (Table 1). The emission band of Ce³⁺ was measured at 370 nm as a rather symmetrical band, while on the excitation spectrum, the following bands have been measured: 190, 220, 245, 281, and 317 nm. The differences in emission and excitation bands for the sample from Dara-i-Pioz (Tajikistan) and the Kipawa River are certainly due to the change in the Ce–O bonding length in both samples. It can be concluded that the crystal-field strength for the Kipawa River specimen is less than for the sample from Dara-i-Pioz. The possibility of Ce⁴⁺ presence and Ce⁴⁺ charge transfer bands on the absorption spectrum at 400 nm, which may be due to the gray color of agrellite crystals, was also assumed [5].

For the studied agrellite sample, the energy transfer between Ce³⁺ and Dy³⁺, Eu³⁺, Sm³⁺, and Pr³⁺ were found. Energy transfer in phosphors with sensitizer-activator ion pairs means that part of the excitation energy of the sensitizer ion transfers to activator ion through a non-radiative process and it subsequently enhances or generates the emission of the activator. The energy transfer among Ce³⁺ and other 4f ions is well known and used to synthesize phosphors with expected properties [12,13,14,15,16,17,18].

Gd³⁺ is another activator with intense absorption and emission lines in the UV range. Its excitation line is usually at 275 nm and corresponds to ⁸S_7/2 → ⁶I_7/2 transition, while emission could occur either from the ⁶I_7/2 or the ⁶P_7/2 level as a 312 nm band. The Gd³⁺ emission was rarely measured for mineral samples. However, the Gd³⁺ luminescence was found in some scheelite, anhydrite, apatite, and fluorite specimens [19]. Moreover, it was found for CaF₂ [19] that due to energy transfer (⁶I_7/2-⁶P_7/2)(Gd³⁺)-(³F₂,³H₆-³H₄)(Pr³⁺), the emission at 275 nm is weakened, whereas at 312 nm it is enhanced. After using laser induction [4], time-resolved measurements of the Gd³⁺ fluorescence for zircon, anhydrite, and hardystonite have been prepared. The energy transfer among Gd³⁺ and many RE³⁺ have been analyzed [10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27]. For the studied agrellite sample, the energy transfer from Gd³⁺ to RE³⁺ could be excluded. The validity of this thesis is based on the fact that:

(a)

for all substances known from existing literature, the Gd₂O₃ content was about 25 mol% or more.

(b)

the excitation and emission lines of Gd³⁺ did not appear in the agrellite spectra at all, although certain narrow lines should be clearly visible.

3.2.1. Energy Transfer Ce³⁺-Dy³⁺

The intense emission of Dy³⁺ related to ⁴F_9/2 → ⁶H_13/2 transition at 575 nm was measured for the most suitable excitation at λ = 438 nm (solid olive line in Figure 9). However, for λ_exc = 321 nm, not only was Ce³⁺ emission at 388 nm measured, but so was a Dy³⁺ emission band (solid red lines in Figure 9), and luminescence intensity was stronger than for the previous excitation. Moreover, on the excitation spectrum at λ_em = 575 nm corresponded to the ⁴F_9/2 → ⁶H_13/2 transition (dash-dot olive line in Figure 9), while the excitation band at 321 nm prevails on the Dy³⁺ band. It appears that the Ce³⁺ excitation band overlapped on the Dy³⁺ 4f⁹-4f⁸5d broadband or that it is transferred to the ⁴K_{15/2, 17/2} excited Dy³⁺ level.

3.2.2. Energy Transfer Ce³⁺-Sm³⁺, Pr³⁺, and Eu³⁺

The efficient emission of Sm³⁺ and Pr³⁺ was also found for agrellite crystal. In the spectral range of 550–700 nm, the emission lines of these two ions are located close to each other and are: at 562 nm and 569 nm as ⁴G_5/2 → ⁶H_5/2 transition of Sm³⁺; at 601 nm and 607 nm of both Sm^{3+ 4}G_5/2 → ⁶H_7/2 and Pr^{3+ 1}D₂ → ³H₄ + ³P₀ → ³H₆ transitions; and at 648 nm mainly for Pr³⁺ transition ³P₀ → ³F₂. As it was found earlier [9], the most convenient excitation for Sm³⁺ is the 399-402 nm line, while for Pr³⁺ it is 480 nm (Figure 10).

For λ_exc = 402 nm excitation, beside of Sm³⁺ and Pr³⁺ emission lines, the Eu³⁺ emission at 613 nm attributed to transition ⁵D₀ → ⁷F₂ have been measured as well (Figure 11).

No emission of Eu²⁺ was found. The emission of Eu³⁺ as a characteristic line at 613 nm was obtained for the most convenient excitation λ_exc = 393 nm (Figure 12). However, this excitation also caused Sm³⁺ and Pr³⁺ emission.

Under excitation λ_exc = 321 nm, not only the emission of Dy³⁺ (574 nm) but also of Sm³⁺ and Pr³⁺ (562 nm, 569 nm, 601 nm, 607 nm), was measured (Figure 9). On the excitation spectra monitored at 562 nm, not only the most convenient bands for Sm³⁺ (and Pr³⁺) were measured but an intense band of Ce³⁺ was recorded too. In the excitation spectra of Sm³⁺, Pr³⁺, and Eu³⁺, the Ce³⁺ band at 321–323 nm always appeared and it was intense. (Figure 13). This same effect has been measured when luminescence was excited into the Ce³⁺ emission band, i.e., 388 nm (Figure 14).

The presence of the Ce³⁺ excitation band in the PLE spectrum of Dy³⁺, Sm³⁺, Pr³⁺, and Eu³⁺ indicated the occurrence of Ce³⁺ → (Dy³⁺, Sm³⁺, Pr³⁺, and Eu³⁺) energy transfer process. Upon UV irradiation, electrons from the ²F_5/2 ground state of Ce³⁺ are excited into the 5d excited state. Some of these electrons return to the ground states (²F_7/2 and ²F_5/2) of Ce³⁺ ions, resulting in the violet-blue emissions of Ce³⁺ due to the 5d → 4f transition. At the same time, owing to the nonradiative resonant energy-transfer, a portion of excited electrons can be transferred into the ⁴K_{15/2, 17/2} excited levels of Dy³⁺ and then relax, mainly as ⁴F_9/2 → ⁶H_13/2 transition. Similarly, another portion of excited electrons can be transferred into excited levels of ⁴K_11/2 or ³P₂ of Sm³⁺ or Pr³⁺ and subsequently, the electrons can relax to their ground levels.

4. Conclusions

In the studied agrellite crystal, the lanthanide ions form a complex arrangement of luminescence centers. The f-f emission of many ions has been distinctly excited by the excitation band of Ce³⁺. Generally, the mechanism of energy transfer from donor to acceptor ions can be attributed to an exchange interaction or electric multipolar interaction. The average distance R_c between the Ce³⁺ donors and Sm³⁺ or Pr³⁺ acceptors was estimated as a value according to the below equation where C is the total concentration of 4fⁿ ions in the studied sample, N is the coordination number, and V is the cell volume:

$$R_c=2\sqrt[3]{\frac{3\cdot V}{4\pi \cdot C\cdot N}}$$

when this value was determined to be 19.38 Å and it is greater than 5 Å, the energy transfer process would take place via electric multipolar interaction. The direct confirmation of this conclusion could be obtained by measuring the decay lifetimes of Ce³⁺, Dy³⁺, Sm³⁺, and Pr³⁺ emissions, as well as the changes in luminescence intensities of these ions for samples containing their variable content. It is necessary to carry out measurements for other specimens of agrellite, differing in the content of these ions.

Contrary to the information on the website [3], the luminescence of Mn²⁺ has not been observed, although it was stated at 580 nm; similarly, no emission band of Eu²⁺ which was notified at 410 nm was observed. Additionally, the emission lines of Dy³⁺ at 478 nm or of Er³⁺ at 522 nm were not clear. It was observed that the measured emission spectra in the 400–560 nm range show intensive scattering of incident radiation not related to the effect of light reflection from the crystal surface. It can be assumed that it is due to the presence of numerous defects on [Si₄O₁₀]⁴⁻ and will be confirmed after femtosecond excitation measurements.

Some differences have been observed between the emission spectra of agrellite specimens for various localities presented in [4] or [5] and our results. The main reason for these differences is certainly the difference in REE content, although the data contained in [1,2] are incomplete. The agrellite specimen from the Kipawa River studied in the current research contains more Ce and a comparable amount of Mn and Fe as a sample from Dara-i-Pioz, and it is more than the sample from Yakutia [5]. However, contrary to [1] but similar to [5], we did not measure the emission of Mn²⁺ as was stated for the specimen from Dara-i-Pioz, and also the emission of Fe³⁺_tetra and Eu²⁺ as was shown for the specimen from Yakutia. Compared to previous luminescence results for specimens from the Kipawa River [4], the emissions from Er³⁺, Pr³⁺, and Eu³⁺ were also measured in the current research.

The most important result of the present research is the demonstration of the phenomenon of effective energy transfer between Ce³⁺ as a donor and Pr³⁺, Sm³⁺, and Dy³⁺ as the acceptors. For this reason, a synthetic material based on agrellite, subsidized with the right amount of these ions, can be an efficient white light emitter.